The present invention relates to compression release mechanisms for internal combustion engines which operate a valve at low engine speeds to release pressure within the engine cylinder during the compression portion of the combustion cycle.
It is desirable in internal combustion engines to reduce the force required to turn over the engine during starting. It is particularly advantageous to reduce the starting forces in small internal combustion engines which are to be started by hand. In addition, such hand started engines must provide a mechanism to eliminate the danger of physical injury from engine kickback.
The chief cause of difficulty in turning over an internal combustion engine is the engine compression. The prior art is replete with mechanisms for releasing or reducing compression during starting. Early devices provided a manually operated valve which released the pressure from the cylinder during starting. The disadvantage of such a manual valve is that it must be quickly closed by the operator after cranking in order for the engine to start. The manual operated valve requires a certain amount of skill in order to properly start the engine and is susceptible to operator oversight. The prior art also teaches a variety of automatic compression release mechanisms which are governed by the speed of the engine. At low engine speeds the compression release mechanism opens a valve during the compression portion of a combustion cycle. When the speed increases above a given amount, the compression release mechanism no longer operates to open the valve during the engine compression.
Many of the prior art devices utilized the existing exhaust valve within the engine to release the compression during engine starting. In this type of a device, the compression release mechanism operated in conjunction with the cam shaft on which a valve lifter for the exhaust valve rode. An example of this type of mechanism is shown in U.S. Pat. No. 3,362,390. This device has a crescent shaped flyweight which allows a latching pin to pivot less than 90.degree. into different positions depending upon engine speed. In one position, the latching pin engages a valve lifter raising the lifter from a cam surface during engine compression. In prior mechanisms of this type, the lifter dropped off the pin back onto the cam surface at the end of the compression portion of the engine cycle. This abrupt transition generated additional noise in the engine. Furthermore, the latch pin was not rigidly held by the flyweight in its normal operating position thereby allowing the pin to move back and forth.